Conventional apparatuses, in particular converter circuits, as are used nowadays to compensate for voltage dips in the voltage of an electrical single-phase or polyphase AC voltage supply network, as may occur on account of short circuits or load changes, are customarily connected in between the voltage source or the voltage sources of the electrical AC voltage supply network and an electrical load. Such an apparatus for the voltage maintenance of an electrical AC voltage supply network is specified in U.S. Pat. No. 5,099,410, for example, wherein a partial converter system is specified which has a first branch pair and a second branch pair connected in parallel therewith, each branch pair being formed from two series-connected driveable power semiconductor switches with in each case a diode reverse-connected in parallel with each power semiconductor switch. The junction point of the power semiconductor switches of the first branch pair furthermore forms a first terminal of the partial converter system, the electrical load usually being connected to the first terminal. Furthermore, the junction point of the power Semiconductor switches of the second branch pair forms a second terminal of the partial converter system, to which the voltage source of the electrical AC voltage supply network is customarily connected. Moreover, an electrical energy store formed from two series-connected capacitors is provided.
The apparatus according to U.S. Pat. No. 5,099,410 is embodied for a single-phase electrical AC voltage supply network and serves, on the one hand, for voltage boosting, in particular for voltage doubling for feeding the electrical load and, on the other hand, as an apparatus for the uninterruptible feeding of the load, the partial converter system being driven, in the event of a fall in voltage of the voltage source, in such a way that essentially the rated voltage of the voltage source is established at the load connected to the first terminal of the partial converter system.
What is problematic in the case of the apparatus in accordance with U.S. Pat. No. 5,099,410 is that although, in the event of a fall in the voltage of the voltage source, uninterruptible feeding of the electrical load is made possible by voltage backup, this backup of the dropped voltage is possible only for a short time of a few milliseconds. Although this time can be lengthened by designing the electrical energy store to have a large capacitance or by means of an additional energy store in the form of a capacitor or a battery, such a design of the energy store or the provision of an additional energy store causes a considerable space requirement and an increased outlay on material, mounting and busbar arrangement.